xvi Annual Address. | February, 1919. 
Expressed in the above terms, the geological time-scale is 
essentially a relative one, and gives no indication of absolute 
duration in years. Since the middle of the nineteenth century, 
many attempts have been made, with widely differing results, 
to translate that scale into years. More than fifty years ago, 
Lyell suggested 250,000,000 vears as the age of the earth. 
Other estimates were as low as 10 or 12 million, the latter based 
chiefly on physical arguments bearing on the age of the sun as 
deduced from its present temperature ; and towards the close 
of the nineteenth century, Lord Kelvin’s figure of 40 million 
years was widely accepted as the maximum possible. Other es- 
timates were based on the rate of denudation and deposition of 
sediment, as measured by the amount of material carried in 
suspension by the great rivers of the present day, and on this 
basis Prof. Sollas arrived at a period of 80 million years, the 
time required for the deposition of the sedimentary rocks, the 
was no longer considered improbable on physical grounds. 
-being apparently lead. “It is further possible to ascertain the 
amounts of th 
lead, occurring in any uranium-bearing mineral and also to 
the parent element, takes place; from these data the period 
uring which decay has been taking place, that is to say, the 
time which has elapsed since the uranium-bearing mineral 
was formed, can be calculated. In this way, minimum ages 
